Optical signals used in a long-distance wavelength multiplexed optical communication system are required to have small wavelength chirp in order to suppress the influence of the fiber dispersion effect. Such optical signals are usually generated by a configuration combined with a laser diode light source and an external modulator. A typical external modulator of this kind is an LN modulator fabricated with LiNbO3 (LN) waveguides. The operation principle of the LN modulator is to couple an optical waveguide with an electrical waveguide, inducing a change in refractive index based on the electro-optic effect by an electrical signal input and creating a phase change in the optical signal. Such LN modulator includes not only a simple optical phase modulator, but also an optical intensity modulator composed of a Mach-Zehnder interferometer, and a device operating as a highly-functional optical switch with numbers of waveguides combined, etc.
Also, there exists a semiconductor optical modulator using the same principle of operation as the LN modulator. There are, for example, a GaAs optical modulator in which a Schottky electrode is disposed on semi-insulating GaAs and formed as an opto-electronic waveguide, an InP/InGaAsP optical modulator configured to apply a voltage more effectively to the core portion of the waveguide along with the optical confinement by using a hetero pn-junction, and the like. While semiconductor optical modulators have an advantage of a small size, there is a problem that the driving voltage is high in both the GaAs optical modulators and the pn-junction type InP/InGaAsP optical modulators.
Recently, what has been proposed as a structure for avoiding such problems is an npin-type optical modulator structure in which both InP cladding layers are made n-type and a thin p layer (p-type barrier layer) is inserted between the two n layers as a barrier layer for suppressing an electron current (Patent Document 1). This npin-type allows the use of a relatively long waveguide because no p-type cladding layer having large optical loss is used. Furthermore, because it has the degree of freedom in that the thickness of a depletion layer can be optimally designed at any value, a reduction of a driving voltage and matching between the electric speed and the light speed can easily be satisfied simultaneously, which is an advantage to improving the response speed of a modulator.
However, the npin-type optical modulator has a semiconductor layer structure similar to that of a transistor, and therefore, when finite light absorption occurs, there is a problem that generated hole carriers accumulate in the p-type barrier layer. Due to this phenomenon, the height of barriers decreases which causes what is called the phototransistor behavior. This can cause not only an increase in the electron current across terminals, i.e., a decrease in withstand voltage, but also dispersion in frequency. Thus, what has been proposed is a structure for sweeping out the accumulated holes by locally forming a new p-type layer (Patent Document 2). However, it has a disadvantage in that it is structurally complex.
FIG. 8 shows the structure of a semiconductor optical modulator according to such conventional technology. This semiconductor optical modulator 80 has first n-type electrode layer 82-1 formed on semiconductor substrate 81, on which first n-type electrode 88-1 and first n-type cladding layer 83-1 are formed. On top of first n-type cladding layer 83-1, layers are further stacked in order of first low-concentration cladding layer 85-1, first intermediate layer 86-1, core layer 87, second intermediate layer 86-2, second low-concentration cladding layer 85-2, p-type cladding layer 84, second n-type cladding layer 83-2, second n-type electrode layer 82-2, and second n-type electrode 88-2. For portions of second n-type cladding layer 83-2 and second n-type electrode layer 82-2, regions 89 are formed in which the conductive type is changed from n-type to p-type.
The core layer 87 is configured such that the electro-optic effect works effectively at an operating optical wavelength. Second intermediate layer 86-2 serves as a connecting layer for preventing carriers occurred due to light absorption from being trapped at a hetero interface, and p-type cladding layer 84 serves as an electronic barrier. In the structure of the semiconductor optical modulator shown in FIG. 8, second n-type electrode 88-2 is in contact with second n-type electrode layer 82-2 and p-type regions 89, and has the same electric potential. Therefore, it allows the holes accumulated in p-type cladding layer 84 due to light absorption to flow to second n-type electrode 88-2, thereby enabling the optical modulator to operate at a higher reverse voltage and in a more stable manner.
However, in order to introduce regions 89 in which the conduction type is changed from n-type to p-type, it is required to use techniques, such as the thermal diffusion of Zn and the implantation of Be ions. This makes the manufacturing process complex, which causes an increased cost of the device for manufacturing.
The present invention has been made in view of such problems, and the object of the invention is to provide an npin-type optical modulator that has a high reverse voltage and is simple to fabricate.    Japanese Patent Laid-Open No. 2005-099387    Japanese Patent Laid-Open No. 2005-114868 (FIGS. 1 to 3)